This application is based on Patent Application No. 2000-084421 filed Mar. 24, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an intrinsic Josephson superconducting tunnel junction device and more specifically to an intrinsic Josephson superconducting tunnel junction device using an oxide superconductor including at least bismuth as a constitutional element.
2. Description of the Related Art
Conventionally known superconducting tunnel devices are generally classified into those using a metal superconductor and those using an oxide superconductor.
The superconducting tunnel junction devices using the metal superconductor use a metal oxide in a barrier layer and some of them are already in practical use. However, they have a drawback that because of a very low critical temperature for superconductivity, the environment in which the superconducting tunnel junction devices of this category can be used is significantly limited.
Among the known superconducting tunnel junction devices using the oxide superconductor, particularly those using an oxide superconductor with a critical temperature for superconductivity of 80 K or higher, are (1) a step-edge-type junction device that has a superconductor layer formed over a substrate with a very small step structure and utilizes the stepped portion of the superconductor, (2) a grain-boundary-type junction device using a grain boundary formed in a thin layer of superconductor as a barrier layer, (3) a stacked-type junction device having an insulating barrier layer sandwiched between superconductor layers, and (4) an intrinsic Josephson junction device that uses a natural superlattice crystal structure characteristic of a single crystal of a bismuth-based oxide superconductor.
To allow the superconducting tunnel junction devices to be used widely in the field of electronics generally requires the use of materials not requiring ultralow temperatures. The aforementioned oxide superconductor meets at least this requirement and thus has been under active research and development. Although many test products using the oxide superconductor have been developed, they are not provided in an industrially applicable form.
That is, in the superconductive tunnel junction devices using the oxide superconductor which have conventionally been test-manufactured, the step-edge-type junction device, the grain-boundary-type junction device and the stacked-type junction device have a drawback that their hysteresis curves of current-voltage characteristics are significantly deviated from an ideal characteristic based on the Josephson effect, with the characteristics jumping from a voltage state to a zero voltage state at a large current value, making it impossible to provide necessary characteristics for electronics applications.
As to the intrinsic Josephson junction devices, the proportional relation in the current-voltage characteristic (geometry of characteristic curve) is generally good, but an accurate control of the magnitudes of the critical current value and step voltage has not yet realized. Further, because the natural superlattice crystal structure is used, a plurality of superconducting tunnel junctions are stacked in series. It is therefore not easy to precisely control the number of junctions (stack number), giving rise to a problem that in the electronics applications the devices produced will fail to exhibit electric characteristics as designed.
To solve these problems, Japanese Patent Application Laid-open No. 10-233536 (1998) discloses a superconducting tunnel junction device having a barrier layer made mainly of a composite oxide generally expressed as Bi2Am+1CumO2m+4 (A is at least one kind of alkaline earth element and m is an integer equal to or less than 4). An example of the barrier layer includes Bi2Sr2Ca0.6Y0.4Cu2O8, Bi2Sr2CaCu2O8, Bi2(La0.6Sr0.4)2CuO6, and Bi2Sr2CuO5.
This junction device exhibits a typical Josephson characteristic with a definite hysteresis and has a good superconducting tunnel junction characteristic. It belongs to the stacked-type junction device (item (3) above) having an insulating barrier layer sandwiched between superconductor layers and differs from the category of the intrinsic Josephson junction device (item (4) above). The method for accurate control of the magnitudes of the critical current and step voltage has not been realized and a precise control of the number of junctions (stack number) is not easy. Hence, there still is a problem with this junction device that in electronics applications the devices produced will not be able to exhibit the electric characteristics as designed.
In light of the aforementioned problems, it is an object of the present invention to provide an intrinsic Josephson superconducting tunnel junction device which uses an oxide superconductor not requiring cryogenic temperatures such as liquid helium temperature and which is capable of precise control of the magnitudes of critical current and step voltage necessary for electronics applications and has good as-designed characteristics.
To solve the aforementioned problems, the inventors of this invention have conducted many years of research and unexpectedly have found that the use of an oxide superconductor layer doped with a particular amount of rare-earth element, particularly yttrium (Y), is effective in limiting to the layer of oxide superconductor the intrinsic Josephson effect observed in bismuth (Bi)-based oxide superconductor. This has led to the present invention.
First, according to the first aspect, the present invention provides an intrinsic Josephson superconducting tunnel junction device that includes an oxide superconductor defined by a general expression (I): Bi2xe2x88x92zPbzSr2Can(1xe2x88x92x)RnxCun+1O2n+6 (nxe2x89xa71, 0 less than xxe2x89xa60.2, 0xe2x89xa6zxe2x89xa61.0, R: rare earth element).
A second aspect of the present invention provides an intrinsic Josephson superconducting tunnel junction device wherein R in the general expression (I) is yttrium (Y).
A third aspect of the present invention provides an intrinsic Josephson superconducting tunnel junction device wherein, in the first or second aspect, n in the general expression (I) is 1.
A fourth aspect of the present invention provides an intrinsic Josephson superconducting tunnel junction device wherein, in the second or third aspect, a layer of the oxide superconductor is stacked at each side with other oxide superconductors.
A fifth aspect of the present invention provides an intrinsic Josephson superconducting tunnel junction device wherein, in the fourth aspect, the other oxide superconductors are Bi-based oxide superconductors having Bi, Sr, Ca, Cu and O as its main components.
A sixth aspect of the present invention provides an intrinsic Josephson superconducting tunnel junction device wherein, in the fifth aspect, the Bi-based oxide superconductor has a composition expressed by a general expression (II): Bi2Sr2Ca1Cu2O8.
A seventh aspect of the present invention provides an intrinsic Josephson superconducting tunnel junction device wherein, in any one of the first to sixth aspect, a structure of the junction device is a mesa structure.
According to this invention, by using a layer of an oxide superconductor doped with a particular amount of rare-earth element such as yttrium to limit the intrinsic Josephson effect observed in a bismuth-based oxide superconductor to a restricted layer of oxide superconductor, it is possible to realize a superconducting tunnel junction device which can precisely control the magnitudes of critical current and step voltage necessary for electronics applications where the device needs to be able to operate without requiring cryogenic environments, and which has good characteristics as designed.
Further, the characteristics of the superconducting tunnel junction device can be controlled by adjusting the amount of rare-earth element doped, such as yttrium (Y); the number of stacks of the superconducting tunnel junction can be controlled by only the thickness of a thin film doped with yttrium (Y) which forms a hetero-structured film; and these characteristics do not depend on the height of the mesa structure. These features provide another advantage that the mesa structure can be fabricated with much greater ease than in the conventional method.
Further, when oxide superconductors other than the bismuth-based oxide superconductor are used for the electrode/wiring materials, there is an advantage that the oxide superconductors forming the superconducting tunnel junction device and the electrode/wiring portions can be formed integral, simplifying the electronic circuits using the superconducting tunnel junction and facilitating the manufacture of the electronic circuits.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.